Transaction Processing: Concepts and Techniques [0] by Jim Gray and Andreas Reuter. Not so much relational algebra, but everything you need to know to implement a classic relational engine.

[0] https://www.amazon.com/Transaction-Processing-Concepts-Techn...

The original paper[0] on relational databases by Edgar Codd is quite accessible.

I also loved the MOOC by Prof. Jennifer Widom[1]

[0] https://www.seas.upenn.edu/~zives/03f/cis550/codd.pdf

[1] http://online.stanford.edu/course/databases-self-paced

In addition to the great links from siblings, it might be interesting to read this Stonebraker paper on the cyclical nature of databases and how, ultimately, they all reduce to a few core paradigms. [0]

[0] http://pages.cs.wisc.edu/~anhai/courses/764-sp07-anhai/datam...

This java project gives you a glimpse at how it all works. It was extracted out of a now defunkt db vendor (I think?). https://calcite.apache.org/

I really liked 'Database in Depth' by C.J. Date.

anything from C. J. Date